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FIgure 1. Panderichthys has no neck, but closely related Tiktaalki does have a neck.

One of the main differences between fish and tetrapods,other than the transition from fins to feet, is the origin of the neck. famously in the amphibian-like fish, Tiktaalik (Fig. 2). The proximal outgroup taxon in the large reptile tree (LRT, 1016 taxa), Panderichthys (Fig. 1), does not have a neck. The skull and opercular bones are jammed up against the cleithrum (pectoral girdle) permitting no wiggle room. That wiggle room ultimately comes from the disappearance of those opercular bones.

Figure 2. Tiktaalik had a neck, that small space between its skull and pectoral girdle is not seen in more primitive taxa, which retain opercular bones, lost in Tiktaalik.

Figure 3. Colosteus relatives according to the LRT. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists. Note the lack of a neck in Osteolepis, Pholidogaster and Colosteus.

The first tetrapod clade,(Fig. 9) with flat-headed Greererpeton at its base, had a neck, though not much of one. In related taxa like Gerrothorax (Fig. 4), the skull and torso were so wide that a neck would have been useless for lateral movements, but essential to help the skull rise during feeding (famously, like a toilet bowl lid!). More derived taxa in this clade, like Metoposaurus, had a little more neck represented by more space between the skull and pectoral girdle.

Figure 4. Gerrothorax, has a wide skull and wide torso permitting little to no lateral skull movement, but vertical movement is not impeded.

The second tetrapod clade,(Fig. 9) with Ossinodus and Acanthostega (Fig. 5) at its base, likewise did not have much of a neck. Perhaps there was less of a neck than in more basal Tiktaalik. This is a small clade with just these two members, so far.

Figure 5. Acanthostega does not have much of a neck. There is little wiggle room between the skull and pectoral girdle.

The third tetrapod clade,(Fig. 9)with Pederpes and Crassigyrinus (Fig. 6) at its base likewise had very little wiggle room between the skull and cleithrum. Crassigyrinus had a short neck between its cheeks, so likely was immobile. In this clade derived members, Sclerocephalus and Eryops, document the third appearance of the neck in tetrapods. Even so, it was a very short relatively immobile neck.

Figure 6. Crassigyrinus has little to no neck. What neck it has is now tucked between its cheeks.

The fourth tetrapod clade(Fig. 9) with Ichthyostega(Fig. 7) as its base, might have had some wiggle room between the skull and tall cleithrum. Not sure whether the small skull or large skull is correct. Certainly its phylogenetic successor, the reptilomorph Proterogyrinus (Fig. 8), had a substantial neck as did most of its descendants (but see below for notable exceptions).

Figure 7. Not sure which is more correct, but this Ichthyostega data shows little to no wiggle room for the larger skull, much more for the smaller skull.

Basal reptilomorphaand in the clade Seymouriamorpha, like Seymouria, and in the LRT leads to both Reptilia and Lepospondyli, had an increasingly mobile neck.

Figure 8. Proterogyrinus had a substantial neck apart from the pectoral girdle.

The number of cervicalsremains low (under 4) in basal lepospondyls, and sometimes that number decreases to one. An exception, Eocaecilia, had 5 elongate cervicals. Basal amniotes, like Gephyrostegus, had six flexible cervicals.

Figure 9. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Yesterday we looked at Entelognathus (Figs. 1-3; Zhu et al. 2013), a Silurian placoderm transitional to bony fish. That was my first placoderm and I made some errors that have since been corrected. Those errors were corrected when I realized the frontal (pineal in placoderms and Cheirolepis) originated as a tiny median (purple) triangle that included the pineal opening. I was also confused by the splitting of the parietal in Osteolepis, which I thought gave rise to the parietal/postparietal split, but instead that is an autapomorphy arising only in certain Osteolepis specimens. Further confusion comes from the fusion of bones, the splitting of bones and the different names given to the same bone in Silurian to Devonian taxa. Because of this, today and today only I will call the bones by the colors provided by Zhu et al. A key to their various names is provided (Fig. 1).

I was also surprised
to see that Zhu et al. 2013 found no trace of a purple/orange division in Entelognathus (Fig. 1f). This is odd for a transitional taxon, but still possible. Worth looking into. Equally odd, Zhu et al. did not color the purple bone consistently (Fig. 1).

The pineal opening drift
from the purple to the orange bones attends the lengthening of the rostrum and perhaps the brain and olfactory regions. The purple bone invades the paired orange bones and at the posterior tip of the parietal is the pineal opening. So the purple bone more or less delivers the pineal opening more or less in the middle of the orange bones.

Figure 1. From Zhu et al. 2013 SuppData showing placoderm and other basal vertebrate skull roofs. Note: Entelognathus is the only taxon without a frontal/parietal split, which I found in the photos of the fossil, figure 2 and corrected at the tip of the long arrow.

I traced bone sutures on photos of the specimenand found that purple/orange division. So now Entelognathus has a complete set of skull roofing bones from the nasal to the frontal to the parietal and post parietal. I may have even seen where the yellow green intertemporal splits from the orange parietal.

I hope to never do another fish.
But happy that I was able to resolve some earlier questions and move on. Feelings aside, mistakes that go on unnoticed are worse than mistakes you, or others, find and correct.

Images repaired May 18, 2017 after studying photos of the specimen, comparing related taxa and dispensing with false paradigms. Click here for more details.

Barford 2013 wrote: “It may be hard to see, but you seem to share a family resemblance with Entelognathus primordialis. The fish, which lived 419 million years ago in an area that is now part of China, is the earliest known species with a modern jaw.”Here (Fig. 1) one can identify a complete set of homologous tetrapod skull bones understood by the original authors, who identified the bones with traditional placoderm names. (Ala, placoderms, bony fish and sacropterygians, including tetrapods, have different names for the same bone). And they made a mistake or two along the way, none of which negate their conclusions, but cement them.

I never thought I’d be featuring any placoderm fish in this blog
or in ReptileEvolution.com, but Entelognathus, as everyone already knows — and I just learned, is something very special. A major discovery. And this was my first day studying placoderms.

Barford 2013 reported,“Palaeontologists have traditionally believed that the fishes’ features bore no relation to ours. They assumed that the placoderm face was lost to evolutionary history, and most thought that the last common ancestor of living jawed vertebrates had no distinct jawbones — that it was similar to a shark, with a skeleton made mostly of cartilage and at most a covering of little bony plates. The theory went that the bony fishes evolved later, independently developing large facial bones and inventing the ‘modern’ jaw. Such fishes went on to dominate the seas and ultimately gave rise to land vertebrates. [Entelognathus] has what looks like a bony fish’s jaw, even though it is older than the earliest known sharks and bony fishes.”

More than that,all of the skull bones find homologies in tetrapods and bony fish (Figs. 1, 2) when certain bones are correctly identified or homologized. It just takes a few colors here and there to make it all clear.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added. This revised image adds a small triangular frontal between the anterior processes of the parietal and the rest of the bones follow suit.

All of the bones in the skull of Entelognathusfind homologies with those in Cheirolepis (Whiteaves 1881; Fig. 2) and also with tetrapods. Entelognathus lived 59 million years before the appearance of tetrapods like Ichthyostega. and is someday going to be a part of the story behind those Middle Devonian footprints.

On the subject of nomenclatureZhu et al. 2013 (SuppData) list the various names given to fish skull bones and their homologies in other fish clades. Some of the more confusing include:

The parietal in sarcopterygians is the frontal in actinopterygians and the preorbital in placoderms.

The postparietal in sarcopterygians is the parietal in actinopterygians and the central in placoderms.

The supratemporal in sarcopterygians is the intertemporal in actinopterygians and the marginal in placoderms.

The tabular in sarcopterygians is the supratemporal in actinopterygians and the anterior paranuchal in placoderms.

And there are others…

Where is the authority that can fix this problem?
But if we fix it, then what? Then all prior literature will have to be translated. Either way, we’re hosed. Maybe we should just colorize homologous bones and leave it at that, as Zhu et al. did in their SuppData.

Entelognathus precedes Cheirolepis by 29 million years.
Preopercular and opercular bones do not appear in Entelognathus, but are present in Cheirolepis. So they are new bones in osteichythys.

The ‘al’ bone in Entelognathus (Fig. 1) is the cleithrum, supporting the pectoral fin.

The split (spiracle) between the skull roofing bones (intertemporal. supratemporal, tabular) and cheek bone (squamosal) do not appear in Entelognathus, but do so in Cheirolepis.

Sclerotic rings are not necessary in such small and well-protected eyes as in Entelognathus and if present, would have been very tiny and fragile.

Comparisons of the circumorbital bones in Entelognathus and Cheirolepis are strikingly similar down to the small post-orbit depression in the jugal in Entelognathus that becomes a notch in Cheirolepis.

Comparisons of the postopercular bones
of Cheirolepis and Osteolepis (Fig. 2) show little to no homology, suggesting a possible separate but convergent derivation.

Note some skull bones
later split apart at the median, while others fuse together. It’s their shapes and locations that identify them. “The large hexagonal central plate seems to have a single ossification centre, whereas most placoderms have paired centrals,” reports Zhu et al, making a case in point. A pineal opening is not present in the pineal plate (fused parietals) of Enteleognathus. This is further evidence that the pineal opening migrated from the frontals to the parietals over tens of millions of years. More on that tomorrow.

Barford 2013 concludes“There remains a chance that E. primordialis evolved its jaw independently from the bony fish, so that we did not inherit it, and the resemblance is an illusion.”I don’t agree with this conclusion. The evidence for homology elsewhere overwhelms any competing hypotheses.

Friedman and Brazeau (2013) also comment on this discovery.“First, Entelognathus always branches outside the radiation of living jawed vertebrates, meaning that key components of the osteichthyan face are no longer unique innovations of that group. Second, acanthodians — that pivotal assortment of extinct shark-like fishes — are shifted, en masse, to the branch containing the cartilaginous fishes. This triggers a cascade of implications. If all acanthodians are early cartilaginous fishes, then their shark-like features are not generalities of jawed vertebrates, but specializations of the cartilaginous-fish branch. The most recent common ancestor of jawed vertebrates was thus probably clad in bony armor of the sort common to both placoderms and bony fishes. This inversion of a classic scenario in vertebrate evolution raises an obvious question: how did we get it so wrong?”

In summaryEven when someone gets it right, some of the details may still be correctable – and the present corrections do not overturn the conclusion, but support it. As usual, I have not seen the fossil firsthand. I have not added Entelognathus to the LRT. I simply make comparisons to published figures of Cheirolepis, which was one source of the earlier problems I had, no hopefully settled.

Thanks to David M.
for directing me to the Entelognathus paper. : – )

Please let me know
if someone else has drawn the same insight in the last 4 years since the publication of Zhu et al. 2013. If so, I am unaware of it.

Comparing therapsid skulls shows that basal forms, like the gorgonpsids (Fig. 1) had a postfrontal and postorbital. Derived forms (likePachygenelus(Fig. 1) had neither. You can see the jugal rise in certain cynodonts, taking over where the postorbital retreated. You can also see either the fusion of the postfrontal and postorbital, or the disappearance of the postorbital (but workers like to label the remaining bone the postorbital even though it is largely a postfrontal). As this all goes into scoring, it’s important to that end.

Figure 1. Several gorgonopsids and cynodonts along with a single therocephalian documenting the disappearance of the postorbital and postfrontal. Pink is the pre parietal, absent in cynodonts. Yellow = prefrontal. Green = postorbital. Blue = jugal.

Not much else today. Just wanted to share this and invite comments. Does anyone know the transitional taxon that might clarify this issue? Likely a basal cynodont, like Charassognathus (Fig. 1). Aelurognathus might have documented something on this subject, but the parts are missing from the fossil. I think we’re looking for a small gorgonopsid or therocephalian to show us, something like Regisaurus (Fig. 1), but more primitive.

All this and more from a PhD study by Gebauer (2007).

Updated the next day, February 16, 2014.

Figure 1. The therocephalian Annatherapsidus documenting a small postfrontal and postorbital on a flat skull identifying this taxon as close to the transition at the base of the Cynodontia. This gives us a better chance that the postorbital fused to the postfrontal.

I just discovered a therocephalian originally named Anna and later renamed Annatherapsidus (Fig. 2) that had a reduced postfrontal and reduced postorbital along with wide temporal fenestra and a rather flat skull, both as in Procynosuchus (Fig.1). This appears to be a transitional taxon, the proximal outgroup to the Cynodontia. And this taxon appears to duplicate the pre-fused shape of the postfrontal/postorbital.

ReferencesGebauer EVI 2007. Phylogeny and Evolution of the Gorgonopsia with a Special Reference to the Skull and Skeleton of GPIT/RE/7113 (‘Aelurognathus?’ parringtoni). PhD Dissertation, Eberhard-Karls University at Tübingen. Online here.

The quadratojugal is the cheek bone that connects the jugal to the quadrate in most reptiles. Typically the quadratojugal is ventral to the squamosal. According to the large reptile tree the quadratojugal disappears or nearly disappears (without becoming fused to another bone) and then reappears in the following taxon pairs: